Hereinafter, a “Q” prefix in a word of phrase is indicative of a reference of that word or phrase in a quantum computing context unless expressly distinguished where used.
Molecules and subatomic particles follow the laws of quantum mechanics, a branch of physics that explores how the physical world works at the most fundamental levels. At this level, particles behave in strange ways, taking on more than one state at the same time, and interacting with other particles that are very far away. Quantum computing harnesses these quantum phenomena to process information.
The computers we use today are known as classical computers (also referred to herein as “conventional” computers or conventional nodes, or “CN”). A conventional computer uses a conventional processor fabricated using semiconductor materials and technology, a semiconductor memory, and a magnetic or solid-state storage device, in what is known as a Von Neumann architecture. Particularly, the processors in conventional computers are binary processors, i.e., operating on binary data represented in 1 and 0.
A quantum processor (q-processor) uses the odd nature of entangled qubit devices (compactly referred to herein as “qubit,” plural “qubits”) to perform computational tasks. In the particular realms where quantum mechanics operates, particles of matter can exist in multiple states—such as an “on” state, an “off” state, and both “on” and “off” states simultaneously. Where binary computing using semiconductor processors is limited to using just the on and off states (equivalent to 1 and 0 in binary code), a quantum processor harnesses these quantum states of matter to output signals that are usable in data computing.
Conventional computers encode information in bits. Each bit can take the value of 1 or 0. These 1s and 0s act as on/off switches that ultimately drive computer functions. Quantum computers, on the other hand, are based on qubits, which operate according to two key principles of quantum physics: superposition and entanglement. Superposition means that each qubit can represent both a 1 and a 0 at the same time. Entanglement means that qubits in a superposition can be correlated with each other in a non-classical way; that is, the state of one (whether it is a 1 or a 0 or both) can depend on the state of another, and that there is more information that can be ascertained about the two qubits when they are entangled than when they are treated individually.
Using these two principles, qubits operate as more sophisticated processors of information, enabling quantum computers to function in ways that allow them to solve difficult problems that are intractable using conventional computers. IBM has successfully constructed and demonstrated the operability of a quantum processor using superconducting qubits (IBM is a registered trademark of International Business Machines corporation in the United States and in other countries.)
A superconducting qubit includes a Josephson junction. A Josephson junction is formed by separating two thin-film superconducting metal layers by a non-superconducting material. When the metal in the superconducting layers is caused to become superconducting—e.g. by reducing the temperature of the metal to a specified cryogenic temperature—pairs of electrons can tunnel from one superconducting layer through the non-superconducting layer to the other superconducting layer. In a qubit, the Josephson junction—which functions as a dispersive nonlinear inductor—is electrically coupled in parallel with one or more capacitive devices forming a nonlinear microwave oscillator. The oscillator has a resonance/transition frequency determined by the value of the inductance and the capacitance in the qubit circuit. Any reference to the term “qubit” is a reference to a superconducting qubit circuitry that employs a Josephson junction, unless expressly distinguished where used.
The information processed by qubits is carried or transmitted in the form of microwave signals/photons in the range of microwave frequencies. The microwave signals are captured, processed, and analyzed to decipher the quantum information encoded therein. A readout circuit is a circuit coupled with the qubit to capture, read, and measure the quantum state of the qubit. An output of the readout circuit is information usable by a q-processor to perform computations.
A superconducting qubit has two quantum states—|0> and 1>. These two states may be two energy states of atoms, for example, the ground (|g>) and first excited state (|e>) of a superconducting artificial atom (superconducting qubit). Other examples include spin-up and spin-down of the nuclear or electronic spins, two positions of a crystalline defect, and two states of a quantum dot. Since the system is of a quantum nature, any combination of the two states are allowed and valid.
For quantum computing using qubits to be reliable, quantum circuits, e.g., the qubits themselves, the readout circuitry associated with the qubits, and other parts of the quantum processor, must not alter the energy states of the qubit, such as by injecting or dissipating energy, in any significant manner or influence the relative phase between the 0> and 1> states of the qubit. This operational constraint on any circuit that operates with quantum information necessitates special considerations in fabricating semiconductor and superconducting structures that are used in such circuits.
A microwave isolator is a device that allows microwave light waves to pass through it without significant amplitude attenuation in one direction, and prohibits or significantly attenuates the microwave light waves when attempting to pass through it in the opposite direction. A reference herein to an “isolator” is a reference to a microwave isolator.
In other words, the isolator operates as a microwave light gate, and the response of the device is dependent on the direction in which the microwave light is propagating through the device.
Isolators are used in quantum computing for guiding the microwave signals into and out of the quantum processor in a specified flow direction. The illustrative embodiments recognize that presently used commercial cryogenic isolators suffer from serious problems that greatly limit their applicability and usability in quantum computing. For example, commercially available cryogenic Isolators are large in size, heavy in weight, difficult to thermalize, use ferrites which are difficult to fabricate/integrate on chip, and incorporate magnets which can have negative effects on superconducting circuits. To give some examples, the size of a presently available cryogenic isolator is 8.5 centimeters (cm)×3.1 cm×1.7 cm=45 cm3, and the isolator weighs 229.5 grams (g). The copper bracket that is used to thermalize the isolator weighs 183.1 g. The size of a presently available cryogenic circulator is 4.5 cm×3.5 cm×1.8 cm, and weighs 41.2 g.
A standard one input-one output line setup, which connects one qubit resonator and one quantum-limited Josephson parametric amplifier working in reflection (such as the Josephson parametric converter (JPC)), uses two circulators, and three isolators (two following the Josephson parametric amplifier in order to protect the qubit from noise coming back down the output chain). This setup accounts for a volume of at least 191.1 cm3 and weight of at least 1.5 kg (just from the circulators and isolators). The volume calculation does not take into account the copper brackets that are used for thermalization.
These are large sizes and weights as compared to nanometer-scale Josephson junction in a qubit. Clearly, the presently available isolators and circulators are not conducive to fabrication on semiconductor chips. The magnetic flux due to the incorporated magnets is too strong compared to the femtoTesla (10−15 T) level of magnetic flux that is sometimes used to flux bias certain superconducting quantum systems such as flux-tunable qubits.
The illustrative embodiments recognize that a new isolator design is needed that is more conducive to quantum computing applications.